1. Field of the Invention
This invention relates in general to the art of selective printing apparatus.
2. Brief Description of the Prior Art
Prior art selective printing apparatus have employed unitary endless printing bands having a printing section and a non-printing human readable section with lugs on the underside of the printing band as disclosed in Canadian Pat. No. 653,495. Typical apparatus have printing bands with between about three and seven lugs per inch of band length. Other known printing bands have 331/3 lugs per inch of band length with a diamond knurl on the drive wheel to drive the printing band about a rotatably mounted square support wheel. U.S. Pat. No. 3,418,929 discloses a printing band with twenty lugs per inch. A printing band with no lugs on its underside is disclosed in U.S. Pat. No. 685,474. Another printing band is disclosed in U.S. Pat. No. 3,744,411 as having lugs only on a non-printing section of the printing band; this patent also discloses the provision of abutments to limit the rotation of the drive wheel. Notwithstanding the provision of abutments or other means designed to prevent the user from advancing the band to a position in which the non-printing section is brought to the support at the printing zone, some users may attempt to advance the band to this position forceably by applying undue force to the selector and this can cause damage to the apparatus.
Many of the acceptable printing bands used in commercial printing apparatus have relatively large lugs similar to the ones shown in Canadian Pat. No. 653,495, wherein the printing band passes under tension about a generally annular drive wheel and a square support wheel. To provide lower manufacturing cost for such apparatus, some commercial apparatus have used a stationary support rather than a square support wheel. While the relatively large lugs will effect excellent traction between the printing band and the drive wheel, the tension in the printing band increases each time the printing band is advanced and a lug passes around the stationary support; while the lugs cooperate with the support to provide excellent detenting of the band, the lugs offer resistance to the smooth advance of the band to a different selected position. Moreover, sometimes such lugs have a tendency to shear off after having been advanced repeatedly about the stationary support.